Modern electroplating processes are widely used for the manufacturing of semiconductor parts and devices. Being a part of manufacturing of sophisticated and very highly integrated circuits, these processes require rigorous monitoring. One monitoring system, the Real Time Analyzer (RTA (Technic, Inc., Cranston, R.I.) allows control of electroplating solutions to the extent expected in the highly demanding semiconductor manufacturing. The system performs an in-situ analysis using exclusively electroanalytical techniques for the bath constituents.
The advantages of using electroanalytical measurements to monitor and/or control plating bath solutions include direct (as opposed to indirect, requiring sample pretreatment) analysis and non-invasiveness. Such electroanalytical methods perform activities very similar to those performed by the electroplating processes themselves, but at a significantly smaller scale. Thus, these electrochemical/electroanalytical measurements do not introduce any changes into the analyzed solutions, which can be returned to the bath after analysis. Alternatively, the solution after analysis can be directed to the wastes, if returning it back to the bath is not preferred by the process route.
By directly analyzing the undiluted plating bath solution using such electrochemical methods, the RTA approach provides accurate measurements of each added constituent of the bath and can characterize the plating bath performance while the plating is in process, thereby enabling early fault detection to minimize waste.
Because electrochemical processes (electroplating as well as electroanalysis) are sensitive to temperature variations in the electroplating solution, in order to achieve high stability and reliability of electroanalytical measurements (and the electroplating process thereby), the temperature of the measured solutions need to be maintained at constant level within a narrow tolerance range. In other words, the belief is that tight temperature control is required for semiconductor manufacturing and bath analysis.
Two possible designs for a sampling device allowing plating bath analyses with temperature control are presented in FIGS. 1 and 2. (For simplicity, the electrical connection from the probe to the computerized potentiostat is omitted from these figures.)
FIG. 1 illustrates a simple temperature controlled measurement setup with a closed-loop bath circulation. A vessel containing a solution to be analyzed is submersed into a temperature controlled device. The RTA probe (an electrochemical cell used for the measurements) is submersed in the same device. The solution from the bottle is delivered to the probe by a PTFE membrane pump (required by RTA measurement routines). Since the temperature in the measurement compartment (a bottom part of the RTA probe) and the vessel containing a sample of analyzed solution need to be kept at the same temperature, both pieces need to be submersed as much as possible in the constant temperature device. The temperature controlled device can be any of these: a chilling-heating baths (a classical water bath, for example), a closed air chamber, or devices used for technological processes (in this case electroplating) such as a plating solution reservoir tank. This temperature controlled device may comprise very different approaches and may be modified depending on effectiveness, cost and/or simplicity. It is well known that the stability and effectiveness of liquid temperature controlled devices are much higher than air-based or gas-based devices. On the other hand, it is quite difficult to fully submerse the pump and all tubing in such a liquid-based device.
The liquid-based temperature control devices seem to be much more effective, thus more frequently used. Although the exposure of the tubing, pumps and valves can be minimized, it is not simple to eliminate its negative impact totally. This task is getting even more difficult with the extent of automation required by the applications (the simpler the setup, the easier the way of maintaining constant temperature). These complications are shown in FIG. 2, which illustrates a multi-stream capable temperature controlled measurement setup. Having different stream paths delivering different solutions to the sampling vessel and the probe requires several valves, pumps, and associated tubing. What this means is that more parts are exposed to the environment at a different than required temperature. Because these portions of the setup are exposed to the non-temperature controlled environment, additional temperature variations in the analyzed solution may be introduced during the sometimes quite lengthy (typically 15-40 min.) full analysis cycle. On top of these factors, if different streams deliver solution from different sources (bath controlled solution vs. standard solution(s) that usually does not come from temperature-controlled containers), the integrity of the analytical system can be compromised, i.e., analyses of different samples may be performed at different temperatures compromising the robustness of the measurements.
FIGS. 1 and 2 show two possible general layouts as examples. But by an appropriate combination of pumps and valves, any design can be achieved depending on the requirements for the inbound and outbound streams, and the size of sample that is analyzed.
Although the foregoing device designs are feasible, there is an alternative approach that can eliminate all (or almost all) negative effects that are caused by temperature variation in the RTA probe. In U.S. Pat. No. 7,270,733, we disclosed methods for real time monitoring of the constituent electrolytes in electroplating baths based on the chemometric analysis of voltammetric data, specifically using a number of chemometric techniques including modeling power, outlier detection, regression and calibration transfer for analysis of the voltammetric data obtained for various plating baths. Here, we extend these methods to improve robustness and accuracy of analyses by eliminating the effects of varying temperature (during the measurement time) on bath measurements. As a result, the focus of the design is shifted from building a complex, difficult to maintain, and expensive device with a very tight temperature control, to building a software model that allows for compensation of the varying temperature effects.